Basic usage

The superscript mark to reference a footnote can be manually set. See the example below:

I'm writing something here to test \footnote[10]{footnotes working fine}
several features. You can write the footnote text\footnotemark in its
own line.
\footnotetext{Second footnote}

There are three new commands here:

\footnote[10]{footnotes working fine}

Adds a footnote using "10" as reference mark. Unless you have a good reason to do this, it's not recommended because the footnote counter is not altered and you may end up with two different footnotes with the same mark.

\footnotemark

Prints a foot note mark but without the actual footnote. This is helpful to write the actual footnote text in a new line.

Footnotes with multiple references

We can add several references to a single footnote mark

I'm writing something here to test \footnote{footnotes working fine}
several features. You can write the footnote text\footnotemark in its
own line.
\footnotetext{Second footnote}
For instance\footnotemark I can use the same footnote more than
once\footnotemark[\value{footnote}].
\footnotetext{footnote with two references}

The command \footnotemark[\value{footnote}] inserts a superscript corresponding to the current value of the counter footnote.